In recent years, development of projectors, medical inspection devices, soil analysis devices and the like using a semiconductor light-emitting element having an emission wavelength in the visible light range has been advanced. As a semiconductor light-emitting element having an emission wavelength in the visible light range, conventionally, a GaP-based compound semiconductor has been mainly used. However, the GaP-based compound semiconductor is a semiconductor having an indirect-transition type band structure, and has lower transition probability, which has made it hard to increase the light emission efficiency therewith. In light of this, development of semiconductor light-emitting elements having an emission wavelength in the visible light range is advanced by using a material based on a nitride semiconductor which is a direct-transition type semiconductor.
However, at the present time, a semiconductor light-emitting element capable of emitting light in wavelength region of greater than or equal to 520 nm in particular within the visible light range with higher efficiency has not been realized. FIG. 10 is a graph showing the relationship between the main emission wavelength and the internal quantum efficiency. In FIG. 10, the horizontal axis corresponds to the main emission wavelength, while the vertical axis corresponds to the internal quantum efficiency (IQE). Referring to FIG. 10, it can be seen that the internal quantum efficiency abruptly decreases, when the main emission wavelength exceeds 520 nm. The wavelength region in which the internal quantum efficiency decreases as described above is called a “green gap region,” and the decrease in efficiency in such a wavelength region is problematic irrespectively of the GaP or nitride semiconductor. This leads to the demand of elevating the internal quantum efficiency in the green gap region to improve the light emission efficiency of a semiconductor light-emitting element.
As one of the reasons for the decrease in light emission efficiency, particularly, in the wavelength region of greater than or equal to 520 nm, decrease in recombination probability between an electron and a hole in the active layer caused by an internal electric field (a piezoelectric field) can be recited. This point is now described by taking a nitride semiconductor as an example.
A nitride semiconductor such as GaN and AlGaN has a wurtzite crystal structure (hexagonal crystal structure). Regarding planes of the wurtzite crystal structure, the crystal plane and the orientations are expressed by using fundamental vectors represented by a1, a2, a3 and c, according to the 4 index notation (hexagonal crystal indexes). The fundamental vector c extends in the [0001] direction, and this direction is called “c-axis”. The plane perpendicular to the c-axis is called “c-plane” or “(0001) plane”.
Conventionally, in production of a semiconductor light-emitting element using a nitride semiconductor, a substrate having a c-plane substrate as the main face is used as a substrate on which nitride semiconductor crystals are to be grown. Actually, a GaN layer is grown on this substrate at a low temperature, and further a nitride semiconductor layer is grown above the GaN layer. As an active layer that constitutes a layer contributing to light emission, InGaN which is a mixed crystal of GaN and InN is commonly used.
Here, there is a difference in lattice constant between GaN and InN. To be more specific, regarding the a-axial direction, the lattice constant of GaN is 0.319 nm, and the lattice constant of InN is 0.354 nm. Therefore, when an InGaN layer containing InN having a larger lattice constant than GaN is grown above the GaN layer, the InGaN layer receives a compressive strain in the direction perpendicular to the growing face. At this time, the balance of polarization between Ga and IN having positive charge and N having negative charge is disrupted, and an electric field (a piezoelectric field) along the c-axis is generated. As the piezoelectric field is generated in the active layer, the band of the active layer bends and the degree of overlapping between wave functions of the electron and the hole decreases, so that the recombination probability between the electron and the hole in the active layer decreases (so-called “quantum-confined Stark effect”). As a result, the internal quantum efficiency decreases.
For the purpose of achieving the emission wavelength of greater than or equal to 520 nm, it is necessary to increase the In composition contained in an active layer (particularly, a light emitting layer) so as to realize a band gap energy suited for the wavelength. However, when the In composition is increased, the compressive strain increases, and thus the piezoelectric field increases. This results in further deterioration in the internal quantum efficiency.
As a method for increasing the internal quantum efficiency, for example, Patent Document 1 which will be described later gives considerations about a light-emitting element adapted to prevent occurrences of a piezoelectric field in an active layer, by growing the active layer using a substrate having a (10-10) plane called an m-plane, which is normal to a [10-10] direction, for example.